Fix a double free in the List functions

The code was set up so that it would free the individual items and the data
in `freeListData`, but there was already a Go `defer` to free the data item,
resulting in a double free.

Remove the `free` in `freeListData` and leave the original one.

In addition, move the `defer` for freeing the list data before the error
check, so that the data is also free in the error case. This just removes
a minor leak.

This vulnerability was discovered by:
Jasiel Spelman of Trend Micro Zero Day Initiative and Trend Micro Team Nebula

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

secretservice/secretservice_linux.c

@@ -158,5 +158,4 @@ void freeListData(char *** data, unsigned int length) {
 	for(i=0; i<length; i++) {
 		free((*data)[i]);
 	}
-	free(*data);
 }

secretservice/secretservice_linux.go

@@ -92,12 +92,12 @@ func (h Secretservice) List() (map[string]string, error) {
 	defer C.free(unsafe.Pointer(acctsC))
 	var listLenC C.uint
 	err := C.list(credsLabelC, &pathsC, &acctsC, &listLenC)
+	defer C.freeListData(&pathsC, listLenC)
+	defer C.freeListData(&acctsC, listLenC)
 	if err != nil {
 		defer C.g_error_free(err)
 		return nil, errors.New("Error from list function in secretservice_linux.c likely due to error in secretservice library")
 	}
-	defer C.freeListData(&pathsC, listLenC)
-	defer C.freeListData(&acctsC, listLenC)
 
 	resp := make(map[string]string)